The invention disclosed and claimed herein generally pertains to an improved amplifier for driving a gradient coil in a magnetic resonance (MR) imaging system. More particularly, the invention pertains to a single switching amplifier adapted to drive the gradient coil to produce both rapidly changing and substantially constant gradient fields, as required for a specified imaging sequence.
The magnetic field characteristic of an MR gradient coil, with respect to time, is generally of a trapezoidal shape, with very steeply sloping sides. Accordingly, the current provided by a gradient amplifier to drive the coil must likewise have a trapezoidal-shaped characteristic. During the flat top portion of the trapezoid, i.e., when the drive current is substantially constant at a maximum value, the voltage (I*R) required by the gradient coil is primarily determined by the resistance R of the copper windings of the coil (I* is the complex current). However, during the ramped or sloping portions of the trapezoidal current characteristic, when current is rapidly changing between levels of constant current, an additional voltage proportional to the rate of current change (Ldi/dt) is required by the large inductance L of the coil. The Ldi/dt voltage is typically ten to twenty times the I*R voltage. This generally requires compromises in the design of gradient current amplifiers.
In one prior art approach, a linear amplifier is used. Such amplifier is sized for the maximum Ldi/dt voltage that can be encountered, resulting in very large power dissipation at the low I*R voltages which occur during the flat top portion of amplifier operation.
In another approach, a switching amplifier, also known as a switch mode or pulse width modulated (PWM) amplifier, is used to drive the MR gradient coil. A switching amplifier generally comprises an inverter and a high frequency filter. The inverter generates a train of rectangular voltage pulses which are applied to the filter, which is in series with the gradient coil. The filter removes high frequency components of the output voltage applied to the coil so that the principal component of such output voltage is DC. In an MR gradient application, the filter must be comparatively small, in order to allow fast current slew rates, i.e., rapid transition in current between constant current levels. However, a filter which is sufficiently small to allow rapid current change will tend to be too small to remove ripple from the drive current during flat top or constant current operation. Ripple is a component of the square wave, or switching frequency, generated by the inverter. Ripple disturbance is undesirable during flat top periods of gradient operation, since MR data is generally acquired during these periods. In addition, the bus voltage required for the Ldi/dt voltage results in large switching losses during flat tops, even though the amplifier operates at low duty cycles.
A further approach to improve gradient amplifier performance has been to employ a system with two separate amplifiers, one optimized for I*R voltages, and the other for the Ldi/dt voltage. For example, a linear amplifier, with good high fidelity, may be used for flat top periods of gradient operation and a switching amplifier may be used for ramps or current transition periods. However, this approach requires additional parts and components, and thereby increases costs and tends to reduce reliability.